Nucleic acid is a basic object of research in molecular biology, and nucleic acid extraction is an important and basic step in molecular diagnostics. There are many methods for extracting DNA or RNA from a biological sample, such as the traditional organic solvent extraction method, silica gel mold column adsorption method, magnetic bead separation method, charge method, and the like. The magnetic bead separation method is the one most widely used and most suitable for automation. In the magnetic bead separation method, the surfaces of small particles containing magnetic materials are processed so that they can adsorb a desired substance, and then the magnetic beads are adsorbed and enriched by magnet, a waste liquid is discarded. The enriched magnetic beads are washed in order to further remove the impurities and other unwanted substances, while the biological substance of interest can be retained due to its adsorption on the magnetic beads. Finally, the biological substance enriched on the magnetic beads is released into a desired solution system under certain conditions.
The existing automated nucleic acid extraction apparatus using the magnetic bead separation method has evolved from an automatic enzyme immunoassay analyzer and is generally referred to as a “plate-type” scheme. In this scheme, a typical 96-well plate or modification plate thereof (collectively referred to as “96-well plate” hereinafter) is used as a processing unit, and the reagent and the sample solution are added through a plurality of parallel pipetting tips, after which shock mixing is performed on the whole plate. After incubation is completed, the plate is sent to a magnetic separation position where magnetic adsorption is performed, and any waste fluid is pipetted by a plurality of parallel pipetting tips. The operations are repeated a certain number of times to clear interfering substances other than nucleic acids. Thereafter, elution processing is performed on the 96-well plate on which the clearing is completed to obtain elution products (i.e., nucleic acids dispersed in the eluent). Finally, the elution products are brought to a new 96-well plate by pipetting tips so as to be mixed with PCR (polymerase chain reaction) reagent in order to be used in nucleic acid detection of the next step or in other nucleic acid detection or processing operations.
Because the PCR detection uses the 96-well plate as the test unit, the nucleic acid extraction apparatus generally also uses the 96-well plate as the test unit in each test step. However, cross contamination may occur sometimes during PCR detection, which, as suggested by studies, is related to the use of the 96-well plate as the test unit in each test step in the nucleic acid extraction apparatus. In this plate-type scheme, a plurality of samples is processed in parallel. Because the well spacing and the well depth are small, the reaction solution is prone to enter into and thereby pollute the surrounding wells. Furthermore, the pipetting process and discharging process, which are performed in parallel by a plurality of tips, may likely increase the risk of cross contamination. In addition, the mixing operation performed by shocking the whole plate may also likely cause cross contamination.
Since the exponential increase to the nucleic acid provided by the PCR technology leads to very high test sensitivity and the linear range is very wide, there are very high requirements for cross contamination in the PCR technology. However, the plate-type scheme has important defects as mentioned above in avoiding cross contamination, which is an important factor limiting the application of this scheme in clinical practice. Therefore, nucleic acid extraction apparatuses which can prevent cross contamination are needed.